1. Field of the Invention
The present invention generally relates to a real time data retrieval system and, more particularly, to culture and terrain data compression and reconstruction for digital radar landmass simulation.
2. Description of the Prior Art
Computer image generation (CIG) is used in visual training simulators which present scenes to an observer or trainee to allow the observer to practice some task, such as flying an airplane. In a flight simulator, for example, a three-dimensional model of the desired "gaming area" is prepared and stored on magnetic disk or similar bulk storage media. The visual simulator combines an image generator with an electro-optical display system such as a cathode ray tube (CRT) or similar display. The image generator reads in blocks of three-dimensional data from the disk and transforms this data into two-dimensional scene descriptions. The two-dimensional data are converted to analog video that is presented to the operator or trainee via the display. The generated imagery is meant to be representative of the true scenes that the operator would see out of a window if the operator were actually performing the task being simulated. The generation of the display images is said to be in "real time" which is normally taken to mean 30 frames per second, as in the U.S. television standard. CIG systems are described in detail in the book entitled Computer Image Generation edited by Bruce J. Schacter and published by Wiley-Interscience (1983).
One of the most important aircraft instruments is radar, and radar simulation is an important tool for the training of pilots. There has been much progress in radar in the recent years in terms of higher resolution. Typically, the radar is used for obstacle avoidance, navigation in poor weather, and target acquisition, among other things. Accordingly, a digital radar land mass simulator (DRLMS) has to be able to process the ever increasing amount of land mass data for large gaming areas in real time. Data compression and data retrieval have become a critical area where new techniques and hardware are needed to be developed that are cost effective and support the higher throughput rate required for DRLMS.
U.S. Pat. No. 3,769,442 to Heartz discloses a digital radar land mass simulator wherein the cultural features and prominent terrain features such as ridges and valleys are described by means of a sequence of connected edges. Each edge is defined by the two end positions in x,y,z coordinates and the direction. This edge information is stored in an on-line memory. The real time hardware then interpolates between the end points of the data along the edge. This technique can generate good data compression when the number of edges are long. This technique is only for the encoding of prominent terrain features and does not apply to the compression of a geographical area at a resolution of 30 meters for Defense Mapping Agency (DMA) level II and 100 meters for DMA level I. In a later patent, No. 4,017,985, Heartz discloses a system wherein the terrain is fitted with a number of faces enclosed by edges. The terrain along a radial sweep is calculated by its intersection with the faces. For large faces, the compression ratio is high. However, for high resolution data bases, when the number of faces approaches the number of display pixel elements, the data stored for the faces may exceed the data otherwise stored for each pixel, and the advantage of this compression technique diminishes.
Others have described data compression and reconstruction techniques in digital moving map displays. The requirements for data retrieval, compression and reconstruction are similar between digital moving map displays and DRLMS. As one example, U.S. Pat. No. 4,520,506 to Chan et al. describes a modified boundary/footprint approach for the compression of culture features. The scheme is that the compression of culture including linear and area data, is based upon a line generating technique, knowing the starting and the end point data and the gradient in between. To reconstruct an area knowing the information describing the edges enclosing it, a scan line data can be filled in knowing the end point values defined by the intersections of the scan line with the left and right edges of an area. The area, line and point data are reconstructed in decending priority. Again, the compression technique is to encode the feature data in terms of the end points of an edge. Large compression can be achieved when the lines are long and the surfaces are large.
In essence, the above compression techniques for culture data amounts to generating lines and polygons for data compression. The amount of data compression is directly related to the sizes of the edges and faces of the polygons. These techniques are more suitable to fewer numbers of features in the data base.
On commercially available high performance DRLMS systems, the elevation compression uses polynomials to envelope the gridded elevation data from the DMA. The compressed data are the coefficients for the polynomials. To decompress the data off-line, huge amounts of hardware are needed, contributing to higher equipment costs.
New compression techniques need to be developed to simulate the modern radar requirements of higher resolution imagery. The recent advances in very large scale integrated (VLSI) memory circuits has made possible certain techniques that are memory intensive instead of processing intensive. Consequently, through the application of new compression techniques, the amount of hardware for DRLMS on line processing has been greatly reduced to achieve equal or better fidelity than conventional methods.